Adjustable electrical reactance unit



Oct. 9, 1962 T. BARNES ADJUSTABLE ELECTRICAL REACTANCE UNIT Filed Sept.21, 1959 INVENTOR.

LII 61L YN 73 BARNES BY 2 L 1477'0RN J United rates Filed Sept. 21,1959, Ser. No. 841,301 11 Claims. (Cl. 317-249) The present inventionrelates to improvements in adjustable electrical reactance units, and inparticular relates to a miniature unit having a movable piston-typeelectrode actuated by screw means, for example, a trimmer capacitor.

In many electronic installations, adjustable reactance units of thetrimmer type are widely employed, and it is desirable to provide veryfine and accurate adjustment of the units with a minimum of back-lash.For purposes of fine tuning, it is also desirable to minimize accidentalcreepage or movement of the piston due to shock or vibration. With theconstant tendency toward greater miniaturization of circuits, it hasbecome increasingly desirable to make these reactance units as small aspossible and still maintain the maximum range of reactance.

As the term is used herein, I intend to encompass under the termreactance unit such variable electric units as trimmer capacitors,resistors, inductors and the like. In my prior United States Patent No.2,607,826, issued August 19, 1952, I have disclosed such an adjustablereactance unit in the form of a trimmer capacitor having a springdoadedpiston and screw assembly. In the present invention, I have eliminatedall springs, and employ friction means to produce fine screw adjustment.

In accordance with the present invention, there is provided atrimmer-type reactance unit having a movable electrode in the form of apiston which slides within a hollow housing. The piston is actuated by ascrew which is turnable within an intermediate member, the latter inturn having a screw engagement with the piston. The screw, intermediatemember, and piston all telescope with one another to move the pistonupon turning of the screw.

An object of the invention is to provide an adjustable reactance unit ofthe character described in which the piston and the interior of thehollow housing are made elliptical in shape so as to provide for slidingmovement of the piston without rotation thereof, and in addition providea maximum effective area of reactance between the piston electrode andthe fixed electrode.

Another object of the invention is the provision of an adjustableelectrical reactance unit of the character described in which adjustmentmeans are provided for varying the frictional drag on the screw-threadedelements and thereby produce an optimum degree of fine linear movementof the piston.

Still another object of the invention is the provision of an adjustablereactance unit of the character described incorporating improvedadjustment means for the piston, the adjustment means being soconstructed as to provide the highest approach to linear tuning withultra-fine resolution and a minimum of back-lash.

A further object of the invention is the provision of an adjustablereactance unit of the character described which provides a long travelof the piston with no axial movement of the adjusting screw, andconsequently a decrease in overall length of the unit.

Additional objects and advantages of the invention will become apparentduring the course of the following specification when taken inconnection with the accompanying drawings, in which:

FIG. 1 is a rear perspective View of a trimmer condenser made inaccordance with the present invention;

3,058,042 Fat-rented Oct. 9, 1962 ice FIG. 2 is an enlarged longitudinalsection of the trimmer condenser of FIG. 1 taken through the centerthereof and showing the piston in its fully extended position;

'FIG. 3 is an enlarged section, similar to FIG. 2, but showing thetrimmer condenser with the piston in a fully retracted position;

FIG. 4 is a transverse section taken along line 44 of FIG. 2; and

FIG. 5 is a denser.

Referring now specifically to the drawings, the invention herein isshown applied to a variable condenser of the trimmer type. The condenserhas a hollow body 10 made of a material having a high dielectricconstant and preferably a low coeflicient of thermal expansion, exampleof such materials being quartz, glass, polystyrene or the like. The bodyit) is made of elliptical or oval cross-section, rather than circularshape, for purposes which will be presently explained.

The condenser body It has on its outer surface a fixed electrode 12which may be in the form of a band of conductive metal bonded to saidouter surface, or which may be in the form of a silver layer fired onsaid surface. The hollow condenser body 10 also defines a through bore14 of elliptical shape.

A movable electrode, in the form of a piston 16 is also of ellipticalshape and is sized to fit slidably within the bore 14 of 'body 10. Thepiston 16 has an end wall 19 having a circular opening 18 communicatingwith a bore 20 of circular cross-section and larger diameter which boreextends to the opposite end of said piston 16. The bore 20 is providedwith internal threading 22, and forms with the smaller opening 18, ashoulder 24.

Drive means are provided for sliding the piston 16 longitudinally withinthe dielectric body 10' and relative to the fixed electrode 12, wherebyto vary the capacitance of the condenser. Such drive means includes anintermediate cylindrical member 3il and a drive shaft 32.

The cylindrical member 30 has an enlarged end portion 34 bearingexternal threading 36 which is sized to mesh precisely with the internalthreading 22 of piston 16. The main body portion of cylindrical member30 is sized to fit slidably and rotatably through the circular opening18 in the end wall of piston 16. The cylindrical member 30 itself has anend shoulder 38 which contains a circular opening 40, the lattercommunicating with a bore 42 extending to the opposite end of saidmember 30. The bore 42 contains internal threading 44.

The drive shaft 32 extends slidably and rotatably through the end wallopening 40' of the cylindrical member 30, and has an integral terminalextension 46 of enlarged diameter which is located within the threadedbore 42 of said cylindrical member. This terminal ex tension 46 carriesexternal threading 48 sized to mesh accurately with the internalthreading 44 of bore 42.

The free end of the drive shaft 32 has an integral terminal portion 50of square or other non-circular configuration which is used to mount ahead-piece 52. The head-piece 52 is circular, and has a central aperture54 of non-circular shape, which is sized to receive the noncircularterminal portion 50 of the drive shaft 32, such that the head-piece 52is keyed to the end of said drive shaft 32 and the two members turn inunison. On its outer face, the head-piece 52 has a kerf 56 adapted toreceive the blade of a screw driver or similar tool. The head-piece 52also has a circumferential flange 58 which is employed in mounting saidhead piece.

The drive shaft 32 is supported by an end wall 62 which closes off thefront end of the hollow condenser body 10. The end wall 62 is of thesame elliptical shape as the interior of the condenser body and is sizedto be fitted within its front end and secured by soldering to a frontelevational view of the trimmer consilver band 84 to complete the endsurface of the condenser body =10, preventing the end wall 62 frommoving further into the condenser body interior.

The end wall 62 also has a central circular opening 65 which is sized toreceive the drive shaft 32 snugly but rotatably, and also has on itsouter surface a circular recess 60 which is sized to receive thecircumferential flange 58 of the head-piece 62. The end mountingassembly also includes a cover plate 66 of the same size and shape asthe end wall 62 and its peripheral flange 64,

.the cover plate 66 having a central circular aperture 67 .portion ofthe latter is in turn screwed into the threaded bore 20 of piston 16,whereby to couple the piston 16, cylindrical member 30 and drive shaft32. The piston 16 is then slidably inserted into the through bore 14 ofthe condenser body 10, with the free end of drive shaft 32 projectingout of the condenser body. The end Wall 62 is then mounted on thecondenser body 10 as previously described, with the drive shaft 32extending through the end wall opening 65, and the head piece 52 isattached to the drive shaft terminal portion 50. The cover plate 66 isthen placed in position covering over the circumferential flange 58 ofhead piece 52, and with the main body of head piece 52 extending throughthe circular central aperture 67 of cover plate 66.

The cover plate 66 is secured in mounted position by a pair of screws 68(shown in FIG. which extend into the end wall flange 64. These screws68, when tightened, cause the cover plate to frictionally engage thehead piece flange 58 and therefore slightly bind the headpiece 52. Thisbinding creates a drag on the headthe body assembly. The end wall 62 hasan offset peripheral flange 64 which abuts piece 52 which is transmittedto the attached drive shaft 32, and tends to prevent undesired oraccidental rotation of the drive shaft 32.

In order to prevent backlash and slippage as the condenser is adjusted,and thereby provide more accurate tuning control of the condenser,keyhole-hole slots 70 and 72 are cut into the enlarged end portions 46and 34 of the drive shaft 32, and the intermediate cylindrical member30, respectively. Set screws 74 and 76 are mounted in the end faces ofthe respective terminal portions 46 and 34, and extend into therespective slots 78 and 72. When the set screws 74 and 76 are tightenedor turned inwardly, their ends engage the opposite wall surfaces of therespective slots and the screws exert a spreading force on the slots 76and 72. This spreading force presses the external threads 36, 48slightly out of alignment with the respective meshing internal threads22, 44 and thus creates a drag on the screw threading of the .piston,the intermediate cylinder and the drive shaft.

Since the set screws 74 and 76 are threaded into the rear end surfacesof the drive shaft 32 and cylinder 30, which surfaces face the openend'of the condenser body 10, the screws are accessible for adjustmentby insertion of a screw driver or the like through the condenser openend. In order to assure the smoothest and most eflicient adjustment ofthe condenser, one of the screws 74 or 76 should be tightened slightlymore than the other, in a manner which will be presently described indetail.

The piston or movable electrode 16 is made of theelectrically-conductive material, as is the intermediate cylindricalmember 30, the drive shaft 32, and the'end wall 62. It is desirable thatthis conductive material should have a low coefiicient of thermalexpansion which is similar to that of the dielectric material from whichthe hollow body 10 is fabricated. It has ben found that Invar, aniron-nickel alloy is quite similar in expansion characteristics to thebody material of the condenser, and aca cordingly its use is preferred.The head-piece 52 and the cover plate 66 may be made of insulatingmaterial.

A terminal lug is affixed to the fixed electrode 12 on the outer surfaceof the condenser body 10. A second terminal lug 82 is afixed to aconductive metal band 84 which is in turn aflixed around the front endof the condenser body 10. The band 84 makes contact with the flange 64of the end wall 62.

The terminal lugs 80 and 82 are adapted to be connected in the usualmanner within a circuit, as for exam ple, to circuit lead wires. Thefixed electrode '12 is energized by the lug 80. The movable electrode orpiston 16 is energized by lug 82 through the band 84, the end wall 62,the drive shaft 32, and the intermediate cylindrical member 30. In thefully extended position of piston 16, shown in FIG. 2, said piston is inregistry with the fixed electrode 12 and the unit is in condition formaximum capacitance. In the fully-retracted position of FIG. 3, thepiston 16 is completely spaced from the fixed electrode l2, and thisposition represents the minimum capacitance of the unit.

In describing the operation of the condenser, it will be assumed thatthe condenser parts are in the extended position of FIG. 2, representingthe maximum capacitance of the unit. In order to adjust the condenser,the user inserts a screw driver or similar tool in the kerf 56 of headpiece 52 and rotates the same. As the head-piece 52 is turned, the driveshaft 32 turns in unison.

At this point, it should be mentioned that the set screw 74 is tightenedto a greater degree than the screw 76, so as to impart a greaterfrictional tension on the drive shaft threading 48 and cylinder internalthreading 44 shape, cannot rotate, and this screw action therefore movesthe piston longitudinally, the piston sliding through the hollow body 10toward the right, as viewed in FIG. 2, and out of registry with thefixed electrode 12, decreasing the capacitance of the unit.

As the drive shaft 32 continues to rotate, the piston 16 slides throughthe hollow condenser body relative to the longitudinally immovableintermediate member 30 and drive shaft 32 until its rear edge comes intoregistry with the rear edge of the enlarged end portion 34 ofintermediate member 30. At this point a stop plate 88 affixed to therear edge of the piston as by a screw 90, engages the rear edge of theintermediate member end portion 34 and stops further longitudnialmovement of the piston 16 relative to the intermediate member 30. Theintermediate member 30 thus becomes rigidly coupled to' the piston 16and can no longer rotate. Continued turning of the drive shaft 32 nowcauses meshing of its external threading 48 with the internal threading44 of the stationary intermediate member 30, and the latter is thusmoved longitudinally to the right, as viewed in FIG. 2. As theintermediate member moves longitudinally, it carries with it the piston16 until the latter reached its fully retracted position of FIG. 3, inwhich it is wholly out of registry with the fixed electrode 12, and theunit is at its minimum capacitance.

The fully retracted position is determined by engagement of the end Wall19 of piston 16 with the fixed end wall 62. The piston 16 can no longermove further to the right, andthe sets of screw threading bind toprevent further rotation of the drive shaft 32 in the same direction.

In moving the piston 16 back toward its extended position, the driveshaft 32 is turned in the opposite direction. Because of the differencein tension between screws 7 4 and 76, the intermediate cylinder 30 atfirst turns With opening 18 engages the enlarged end portion 34 of theintermediate member 30. The intermediate member 30 is now restrainedfrom rotating, and the drive shaft 32 turns within said intermediatemember, moving the latter to the left until the piston 16 is carried toits fully-extended position. In this position, the enlarged terminalextension 46 of said drive shaft 32 engages the end shoulder 38 of theintermediate cylinder 30, halting further outward movement.

It should be understood that the the screws 74, 76 may termediate member30 then the piston 16.

It will be observed that the telescoping action of the piston 16,intermediate cylinder 30, and drive shaft 32, with the two sets of screwthreads, permits the use of very fine threading over large totaldistances, and consequently achieves very fine tuning by requiring ahigh degree of rotation of the drive shaft 32 to achieve a relativelyshort longitudinal movement of the piston. At the same time the driveshaft 32 is always longitudinally immovable and never projects from theexterior of the condenser body as is usual in conventional trimmers.Thus, the over-all size of the unit is considerably reduced.

As was previously explained, the elliptical shape of the piston 16 andof the hollow body prevents the piston from turning during adjustmenteven though the piston may have a free fit in the bore of said hollowbody and therefore need not require tight tolerances. This provides forfiner and more accurate adjustment of capacitance. In addition, theelliptical shape of these parts provides for greater electrode area thanwould be afforded by a circular shape with a consequent increase incapacity.

With the construction shown herein, the unit may be easily designed toproduce selected modifications in capacitance adjustment. For example,the sides 10a of the elliptical body 10, corresponding to the shortradius thereof, may be made of lesser thickness than the other portionsof the condenser body, as shown in FIG. 4. In this event, the fixedelectrode 12, instead of being made in the form of a continuouselliptical band, may be made in two separated segments located on thesethinner sides 10a and arranged in tandem. This will produce ultrafinevariations in capacitance.

It is to be understood that While the invention has been describedherein in connection with a trimmer capacitor, it is contemplated thatthe adjustment means may be equally applied to all types of adjustablereactance units having movable electrodes of the plunger type.

A latitude of modification, change and substitution is intended in theforegoing disclosure, and in some in stances some features of theinvention will be employed without a corresponding use of otherfeatures. Accordingly, it is appropriate that the appended claims beconstrued broadly and in a manner consistent with the spirit and scopeof the invention.

What I claim is:

1. A variable electrical reactance unit comprising a hollow body havinga longitudinal bore of elliptical crosssection, a piston of ellipticalcross-section contained in said housing for sliding and non-rotatablemovement therein, and drive means for moving the piston longitudinallyin said housing, said drive means comprising an internally threadedcylindrical bore in said piston, an intermediate member having externalthreading meshing with the internally threaded piston bore fortelescoping movement of said intermediate member with said piston whensaid intermediate member is rotated, said intermediate member alsohaving an internally threaded cylindrical bore, a drive shaft journalledin said hollow body and extending longitudinally thereof, externalthreading on said drive shaft meshing with the internally threaded boreof said intermediate member for telescoping movement of said drive shaftwith said intermediate member relative tightness of be reversed, inwhich case the inwill first travel longitudinally and when the driveshaft is rotated and means for rotating said drive shaft from theexterior of said hollow body.

2. A variable electrical reactance unit according to claim 1 which alsoincludes stop means for limiting longitudinal and turning movement ofsaid intermediate member relative to said drive shaft and relative tosaid piston.

3. A variable electrical reactance unit according to claim 1 in whichsaid intermediate member and drive shaft each have friction means foradjustably varying the friction between the external threading on theintermediate member and the internally threaded piston bore and betweenthe external threading on the drive shaft and the internally threadedintermediate member bore respectively,

4. A variable electrical reactance unit according to claim 3 in whichsaid friction means comprises a transverse slot in said intermediatemember and drive shaft extending through the external threading thereof,and a respective screw engaging each of said slots and adapted to spreadapart the respective threading through which said slot extends.

5. A variable electrical reactance unit comprising a hollow body havinga longitudinal bore, a piston slidably contained in said housing, drivemeans for moving the piston longitudinally in said housing, said drivemeans comprising an internally threaded bore in said piston, anintermediate member having external threading meshing with theinternally threaded piston bore, said intermediate member also having aninternally threaded bore, a drive shaft journalled in said hollow bodyand extending longitudinally thereof, external threading on said driveshaft meshing with the internally threaded bore of said intermediatemember and means for rotating said drive shaft from the exterior of saidhollow body, and adjustment means for selectively varying the tension ofthe external threading on the intermediate member and drive shaft, saidadjustment means comprising a respective transverse slot in saidintermediate member and drive shaft extending through the externalthreading thereof, respective set screws threadedly mounted in saidintermediate member and drive shaft and engaging a wall of the slotthereof, each screw being selectively turnable to spread thecorresponding slot and upset the threading adjacent thereto.

6. A trimmer capacitor comprising a hollow body member of dielectricmaterial having a longitudinal bore of elliptical cross-section, a fixedelectrode on the outer surface of said body member, a movable electrodeof elliptical cross-section slidable in said longitudinal bore, anddrive means for said movable electrode, said drive means comprising athreaded bore in said movable electrode, an intermediate member havingexternal threading meshing with said threaded bore for telescoping ofsaid intermediate member within said piston bore upon rotation of saidintermediate member relative to said piston, a threaded bore in saidintermediate member, a drive shaft journalled in said body member andextending longitudinally thereof, external threading on said drive shaftengaging the threaded bore of said intermediate member for telescopingof said drive shaft and intermediate member upon rotation of said driveshaft relative to said intermediate member, stop means for haltingmovement of the intermediate member relative to the piston when theintermediate member is telescoped therewith, stop means for haltingmovement of the drive shaft relative to the intermediate member when thelatter members are telescoped, and means for adjusting the tension ofthe threading between the movable parts for selectively determining theorder of relative rotation of the parts upon turning movement of thedrive shaft.

7. A trimmer capacitor according to claim 6 in which said stop meanscomprises a stop plate on said piston engageable with the intermediatemember when the latter is telescoped with said piston, and an end wallon said body member positioned to be engaged by said cylin- 7 dricalmember when the latter is telescoped with the drive shaft. 7

8. A trimmer capacitor according to 'claim 6 which also includes a headpiece rotatably mounted on the body member and accessible from theexterior thereof, and means rigidly coupling the drive shaft with saidhead piece for rotation of the drive shaft when the head piece isrotated.

9. A trimmer capacitor according to claim 6 in which said adjustingmeans comprises a respective transverse slot in said intermediate memberand drive shaft extending through the external threading thereof, and arespective screw engaging a wall of each slot and adapted to spread theslot apart and upset the meshing engagement of the threading throughwhich the slot extends.

10. A variable electrical reactance unit comprising a hollow body havinga longitudinal bore, a piston slidably contained in said bore, anintermediate member threadedly engaging said piston for rotation of saidintermediate member to produce longitudinal telescoping movement of saidpiston, a drive shaft journalled in said hollow body and extendinglongitudinally thereof, said drive shaft threadedly engaging saidintermediate member,

11. A variable electrical reactance unit comprising a hollow body havinga longitudinal bore, a piston slidably contained in said bore, anintermediate member threadedly engaging said piston for rotation of saidintermediate member to produce longitudinal telescoping movement of saidpiston, a drive shaft journalled in said hollow body and extendinglongitudinally thereof, said drive shaft threadedly engaging saidintermediate member, and means for rotating said drive shaft from theexterior of said hollow body to produce longitudinal telescopingmovement of said intermediate member, said intermediate and means forrotating said drive shaft from the exterior of said hollow body toproduce longitudinal telescoping movement of said intermediate member,said intermediate member and said drive shaft each having friction meansfor adjustably varying the friction between said intermediate member andsaid hollow body and between said drive shaft and said intermediatemember.

member and said drive shaft each having friction means for adjustablyvarying the friction between said intermediate member and said hollowbody and between said drive shaft and said intermediate member, saidintermediate member and said drive shaft being externally threaded, saidfriction means comprising a transverse slot in each of said intermediatemembers and said drive shaft extending through the external threadingthereof,

and respective screws engaging each transverse slot and adapted tospread apart the external threading through which the respective slotextends. 7

References Cited in the file of this patent UNITED STATES PATENTS

